(a) Field of the Invention
The present invention relates to a nitride phosphor is used in luminescence units, such as monitors, liquid crystal back light sources, fluorescent lamps, and light-emitting diodes. The present invention further relates to compositions of the nitride phosphor and a light emitting device comprising the phosphor.
(b) Description of the Prior Art
Semiconductor-based light emitting devices have been extensively used in recent years, especially light-emitting diodes, which have already been successively developed. Because such light emitting devices are provided with the characteristics of high luminescence efficiency, small size, low power consumption and low cost than conventional light emitting apparatus such as cold cathode tubes and incandescent lamps, thus, they are applicable for use in various types of light sources. Semiconductor-based light emitting devices comprise a semiconductor light-emitting element and a phosphor, in which the phosphor absorbs and converts the light emitted from the semiconductor light-emitting element. The light emitted from the semiconductor light-emitting element and the light converted and emitted from the phosphor are mixed and utilized. Such light emitting devices are applicable for use in various areas, including fluorescent lamps, vehicles lighting, monitors, backlit liquid crystal displays, and the like. In which, white light emitting devices are the most extensively used. Current white light emitting devices are assembled by means of a YAG (yttrium aluminum garnet) phosphor (Y3Al5O12:Ce) with cerium as the active center and a semiconductor light-emitting element emitting blue light. However, using the mixed light emitted from a Y3Al5O12:Ce phosphor combined with a semiconductor light-emitting element emitting blue light, the color coordinates of the mixed light are positioned on the connecting line between the color coordinates of the blue light emitted from semiconductor light-emitting element and the color coordinates of the light emitted from Y3Al5O12:Ce phosphor. Hence, the emitted mixed light is white light deficient in red light, and color rendering properties and color saturation are clearly insufficient. In addition, the preferred region of the excitation spectrum of the Y3Al5O12:Ce and the luminescence region of the semiconductor light-emitting element are inconsistent, thereby causing poor conversion efficiency of excitation light, and a high brilliance of white light source is difficult to obtain. In order to resolve the phenomena of poor chromaticity and low luminescence brightness, YAG:Ce phosphors mixed with red light phosphors have been actively developed in recent years, and the quality of phosphors emitting red light has also been improved to increase luminescence brightness.
However, phosphors able to absorb blue light and emit red light are scarce. Current industrial research and development has primarily focused on nitride and oxynitride phosphors. Insofar as is known by the inventors, such phosphors include Sr2Si5N8:Eu phosphors with europium (Eu) as the active center, CaAlSiN3:Eu phosphors and the sialon phosphor having the general formula: MgSi12-(m+n)Alm+nOnN16-n:Eu. However, because the crystal itself has poor heat resisting properties, thus, the Sr2Si5N8:Eu phosphor has the disadvantages of decrease in brightness and color rendering properties after long term usage. And, although the sialon phosphor itself has no durability problems, however, luminescence brightness of the phosphor is clearly insufficient, and thus not commercially popular. Although CaAlSiN3:Eu phosphors have preferred durability, and provide better brightness compared to sialon phosphors, however, industries are still expecting further improvement in the luminescence brightness of the phosphor, thereby enabling the light emitting device to be provided with higher luminescence efficiency.